Since their discovery, single-walled carbon nanotubes (SWNTs) have generated much interest as one of the best candidates for electronic devices due to their exceptional conductivity and field-effect transistor (FET) behavior. However, an unresolved obstacle to realization of their widespread use in applications relates to control of nanotube electronic properties.
During nanotube growth, the wrapping around and joining of a graphene sheet leads to many possible chiralities. With most growth processes, such as carbon-arc discharge, laser ablation of carbon, and chemical vapor deposition methods, about a third of the nanotube species are metallic (m-SWNTs) and the rest semiconducting (s-SWNTs).
For use as the FET active material, for example, only s-SWNTs are desired, as metallic species contribute unwanted conductance when the nanotubes are applied in transistors and electronic circuits. The poor growth selectivity of s-SWNTs and efficiency of destroying m-SWNTs during growth make efficient post-synthesis separation schemes necessary. Although methods such as alternate current (AC) dielectrophoresis, anion exchange chromatography of DNA wrapped carbon nanotubes, and density gradient centrifugation methods have been successfully employed in separation of m-SWNTs and s-SWNTs, difficulty of scaling up limits their application. Furthermore, due to subtle differences in the properties of m-SWNTs and s-SWNTs, these methods typically do not achieve a sufficiently high purity needed for practical transistors, and a tradeoff between yield and purity is required.
In view of the above, there is a need for an improved method of enriching different species of carbon nanotubes that addresses at least one of the above-mentioned problems.